How to use the Constitutive Law class
The constitutive law behaviour is dealt with in kratos by the use of the class "ConstitutiveLaw", with a public interface defined in the file
which also provides some rather extensive inline documentation (in the form of comments in the code).
By design such file aims to provide a very flexible interface to constitutive law modelling, with the specific goal of maximizing the flexibility in the implementation of complex constitutive behaviours. While such approach provide obvious advantages, it also implies that the API is more complex than what would be strictly needed for very simple constitutive laws.
The objective of current HowTo is to provide a brief introduction to the interface
Through the whole section, the following convenctions will be employed:
voigt notation: - 3D case:
STRAIN Voigt Notation: e00 e11 e22 2*e01 2*e12 2*e02 STRESS Voigt Notation: s00 s11 s22 s01 s12 s02
- 2D plane strain/axisymmetric case (4 stress components)
STRAIN Voigt Notation: e00 e11 e22 2*e01 STRESS Voigt Notation: s00 s11 s22 s01
- 2D plane stress
STRAIN Voigt Notation: e00 e11 2*e01 STRESS Voigt Notation: s00 s11 s01
The constitutive law works on the basis of the total deformation gradient F, defined as
F := D(X) / D(X0)
that is, as the deformation gradient connecting the original and deformed configuration
where the initial position X0 is the one obtained by
const array_1d<double,3>& X0 = node->GetInitialPosition()
and the deformed one by
const array_1d<double,3>& X = node->Coordinates() //must coincide with X = node->GetInitialPosition() + node.FastGetSolutionStepValue(DISPLACEMENT);
The ConstitutiveLaw always returns the total stress. Formulations expressed in terms of strain increments shall store internally the strain stresses from which the increment shall be computed
The constitutive law API is based on the use of an auxiliary "Parameters" data structure, designed to encapsulate the data to be passed to the CL and received from it. The parameters data structure should be initialized using the following constructor:
Parameters (const GeometryType& rElementGeometry
,const Properties& rMaterialProperties ,const ProcessInfo& rCurrentProcessInfo) Thus allowing to encapsulate the pointer to the elemental properties, to the element geometry and to the process info.
The data structure does not contain any internal storage and should be initialized with pointers to memory owned by the caller element. Full documentation of the code can be found in the file constitutive_law.hpp [[https://kratos.cimne.upc.es/projects/kratos/repository/entry/kratos/kratos/includes/constitutive_law.h ]]. For ease, the getter interface, returning a reference to the encapsulated data, is reported here
GetOptions() //returns a reference to a flag container, to be employed in passing options to the CL
GetStrainVector() //INPUT/OUTPUT -- note that F will be used preferentially instead of the input strain GetStressVector()
GetProcessInfo() GetMaterialProperties() GetElementGeometry()
there are additionally the two functions
GetDeterminantF0() //DEPRECATED: please set to 1.0 GetDeformationGradientF0() //DEPRECATED: please set to the IdentityMatrix
that are currently deprecated to simplify the constitutive law usage. Until their complete removal please set them respectively to 1.0 and to the IdentityMatrix of correct size. They will be removed asap.
The "Options" flag represents the fundamental tool in steering the control of the constitutive law behaviour. The interface provides a number of boolean flags that can be passed to the constitutive law:
COMPUTE_STRAIN COMPUTE_STRESS COMPUTE_CONSTITUTIVE_TENSOR ISOCHORIC_TENSOR_ONLY VOLUMETRIC_TENSOR_ONLY TOTAL_TENSOR FINALIZE_MATERIAL_RESPONSE
there are 3 additional ones but are currently deprecated and not reported here.